ST. PETERSBURG, Fla. -- Twins catcher Joe Mauer has been diagnosed with a stress reaction in his left fibula that will sideline him for at least a few days.

Mauer had been dealing with soreness in his left leg for the past four to five days. But Twins manager Ron Gardenhire said the catcher arrived at Hammond Stadium on Thursday morning complaining that the pain had migrated from the inner part of his left ankle to the outside of it into his shin.

An MRI was taken of the leg and revealed the stress reaction, which is considered a precursor to a stress fracture. Mauer's treatment will include a bone stimulator and physical therapy, and he will be re-evaluated on Sunday.

Twins general manager Terry Ryan credited Mauer with his quickness in alerting the team's medical staff to the soreness so that it could detect the stress reaction before it could develop into more of a long-term problem. Ryan said that he's confident that Mauer should be able to return to the lineup sometime next week and be available for Opening Day against the Orioles.

This isn't the first injury of Mauer's career. Last year's American League batting champion tore the meniscus in his left knee during his rookie season of 2004. He missed most of that season, but rebounded the past two campaigns to hit .294 in 2005 and .347 in '06.

The plan now will be to see how Mauer reacts over the next few days to treatment and see if his pain decreases. But while it appears that Mauer will remain on schedule to get some more at-bats before the start of the season, Gardenhire admitted that at first he feared the worst.

"Every time you hear MRI you get a little nervous," Gardenhire said. "It wasn't the best thing to wake up to in the morning after a nice cup of coffee. But they told me it's something that could be taken care of and we'll re-evaluate him after a few days here and we'll let him rest. He'll be fine."